This invention relates to the processing of electrical signals so as to discriminate between signal components of interest and unwanted background signal noise. The invention is particularly (but not necessarily exclusively) applicable to ultrasonic pulse-echo techniques where the background noise is largely attributable to reverberation effects resulting from pulsing of the pulse transmitter coupled with the random electrical noise level of the conditioning electronics.
The reverberation effects accompany the transmitted pulse and decay in an approximately exponential manner, being superimposed on the electrical noise level of the electronics. The echo pulses are likewise superimposed on the electrical noise level and are required to be distinguished from the background noise.
In general, the output signal is gated for a suitable interval of time during which the echo pulses are expected to be available for detection. Such gating will usually eliminate the larger amplitude reverberation effects as these will have decayed substantially before the commencement of the gating interval. Nevertheless, in many cases, the gating interval will embrace some of the reverberation components as well as the electrical noise component.
One approach of achieving discrimination is to impose a fixed level threshold of suitable amplitude and, to avoid spurious signals, the threshold level must be selected to be above the largest amplitude reverberation effects that are likely to occur during the gating interval.
This is subject to a number of drawbacks however: the magnitude of the threshold level may result in loss of low amplitude echo signals; the threshold level may need to be modified if the gating interval is changed or shifted; useful information may be lost in case of signal components not falling within the gating interval; and the reverberation effects vary from one ultrasonic transducer to another with the consequence that the threshold levels may likewise differ for different transducers of the same design.